/Engine/Source/Developer/TextureFormat/Public/Interfaces/ITextureFormat.h
This handles mip chains whether or not they have packed mip tails. NoteGetNumEncodedMips()== OutMipTailIndex + 1
void GetEncodedMipIterators ( const [FEncodedTextureExtendedData](API\Developer\TextureFormat\Interfaces\FEncodedTextureExtendedData) * InExtendedData, int32 & OutMipTailIndex, int32 & OutMipsInTail ) const